The present invention relates to wafer or substrate processing chambers, and more particularly to the control of wafer lift pins in a rapid thermal processing (RTP) chamber used for processing semiconductor wafers or other substrates.
In the semiconductor processing field, various processing chambers are used to perform a variety of processes. These processes can include annealing, cleaning, chemical vapor deposition, oxidation, and nitridation. The processes may be applied under vacuum, under gas pressure and with the application of heat.
In one exemplary system for the thermal processing of semiconductor wafers, the wafer is carried by an edge ring which supports it. The wafer substrate is rotated so that the processes occur evenly over the wafer's surface. Loading and unloading of the wafer is automated. A number of lift pins are accommodated by holes or bores in the chamber bottom located below the wafer and edge ring. The lift pins are movable between retracted and extended positions. In the retracted position, the upper ends or tips of the pins are accommodated within the holes in the chamber bottom so as to be relatively shielded from the processes occurring in the chamber.
After a given wafer has been processed, the rotation of the wafer is stopped, and the lift pins are raised from the retracted position to the extended position. During transit from the retracted position to the extended position, the pins contact the lower surface of the wafer, lift it off the edge ring, and finally elevate it well above the edge ring. With the pins in the extended position, a transfer element can be inserted below the wafer. A typical transfer element is an end effector such as a fork or a blade of a robot external to the chamber. The end effector can be inserted into the chamber through a slit valve and is accommodated by the lift pins by being configured to either go around or between the lift pins. Once the end effector is below the wafer, the pins are lowered from the extended position to a retracted position. During the transit between the extended and retracted positions, the pins deposit the wafer on the end effector and then continue downward to the retracted position. The end effector then can be withdrawn from the chamber where it exchanges the wafer for a new one to be processed. The end effector carrying the second wafer then is inserted into the chamber. The pins are raised from the retracted position to the extended position. When the pins reach the level of the end effector, they contact the underside of the wafer and raise the wafer above the end effector until the pins reach the extended position. The end effector then is withdrawn and the pins again lowered from the extended position to the retracted position. When the pins reach the level of the edge ring, the edge ring contacts the underside of the wafer to acquire it from the pins. When the pins reach the retracted position, the edge ring is rotated, and the processing can be commenced. The time required to process each wafer, from its introduction to the chamber to the introduction of the next wafer, is designated the "cycle" time.
According to one exemplary system, the lift pins are driven pneumatically. The pins are coupled to the piston of a pneumatic cylinder. Upper and lower chambers above and below the piston are connected by an associated valve to atmosphere and to a compressed air source. The piston is raised by actuating the lower valve so that the lower chamber is connected to the compressed air source while actuating the upper valve so that the upper chamber is connected or vented to atmosphere. To lower the piston, the valve states are reversed.
For a given lift pin construction, the speed with which the pins move between the retracted and extended positions (the upstroke) and vice versa (the downstroke) is influenced by the pressure of the compressed air source (typically house compressed air at 60-80 pounds per square inch (psi)) and the particular throttling of the valves which can damp movement of the piston. Because of the throttling and other damping factors, the strokes occur at nearly constant velocity. Since the high cost of the chamber makes time-efficient use desirable, the valve throttling and other parameters should be selected to provide the fastest travel possible without damaging the wafer being handled.
The wafer can be damaged by impact of the pins if the pins move upward at an excessive speed. This can occur when the wafer is held either by the edge ring or by the end effector blade. Additionally, if the wafer and pins are moving downward at an excessive speed, the substrate may be damaged by contact with the blade or edge ring upon transfer. Also, if the pins are moving too quickly prior to reaching the upward extreme of their stroke, the wafer will continue moving upward after the pins have stopped and will then fall back down atop the pins and may be damaged. In one exemplary system, for example, the lift pin stroke is approximately 1.18 inches, and it takes approximately two seconds to get from the retracted to the extended position and vice versa.
Thus, it is desirable to facilitate a more efficient use of the chamber by reducing cycle time without unnecessarily risking damage to wafers.